Method and device for protecting an autotransformer for an aircraft

ABSTRACT

The invention relates to a method for protecting a multi-phase autotransformer for an aircraft, including the steps of receiving ( 100 ) values of current output from (I a , I b ) the first and second phases and a value of current input into (I B ) the second phase; determining ( 101 ), from these received values of the second phase, a value of current representative of the operation (I Fb ) of the second phase; determining ( 102 ), as a function of this determined value and of the value of current output from (I a ) the first phase, a value representative of the homopolar current (I det-a ) flowing in the first phase; comparing ( 104 ) this value of homopolar current (I det-a ) with a first predetermined threshold value (S1) at least during a first predetermined period (D1); and controlling ( 106 ) the values of currents input into (I A , I B ) and/or output from (I a , I b ) the phases as a function of the said first comparison.

The invention relates to methods for protecting autotransformers foraircraft, and in particular in the case of opening of phases upstreamfrom these autotransformers.

The invention also relates to devices for protecting autotransformersfor aircraft, and in particular in the case of opening of phasesupstream from these autotransformers.

It is known that aircraft are provided with numerous on-board equipmentitems that consume electrical energy, even increasingly so.

To generate a sufficient quantity of electrical energy, aircraft aretraditionally equipped with rotating electrical machines configured togenerate electrical energy having an rms voltage of approximately 115 V.

Each one of such rotating electrical machines has a considerable weight.

For obvious reasons of weight gain, aircraft are now being equipped withrotating electrical machines configured to generate electrical energyhaving an rms voltage of approximately 230 V, so as to reduce the crosssections of cables in the electrical system in the aircraft.

The latter machines are replacing or even supplementing the machinestock existing traditionally on board aircraft.

However, a stock of traditional machines compatible with those ofaircraft and therefore configured to generate electrical energy havingan rms voltage of approximately 115 V exists on the ground.

It is more convenient and economical to retain the machine stockexisting on the ground.

Consequently, it is necessary to ensure compatibility between anon-board electrical system (in the aircraft) having an rms voltage levelof approximately 230 V and an electrical system on the ground having anrms voltage level of approximately 115 V.

For this purpose, the rotating electrical machines with which aircraftare equipped are now autotransformers (known as ATUs) configured toensure electrical transformation between 230 V and 115 V a.c. voltagesin reversible manner (also referred to as 230 VAC/115 VAC ATUs).

The autotransformers generally have star-star coupling between theirprimary and their secondary and are consequently connected at theneutral in the primary and in the secondary.

When a phase is open (meaning that it is not being supplied) upstreamfrom a three-phase autotransformer of 230 VAC/115 VAC type, a largedisequilibrium is generated on the downstream consuming system, in otherwords on the 115 V side, since the more the electrical loads demandenergy, the more the voltage on the open phase collapses.

In addition, the autotransformer may continue to supply three-phaseelectrical loads connected to this autotransformer by compensating withthe other two non-defective phases. In this case, the electrical energyflowing in the primary turns and the secondary turns of the twonon-defective phases may be much greater than the nominal energy. Thisabnormal operation may therefore cause overheating of theautotransformer, especially when the electrical loads are demandingmaximum energy.

There are known a method and a device for protecting autotransformersprovided with current sensors disposed on each phase of theautotransformer, upstream from the primary turns.

These sensors are configured to measure the value of current input intoeach phase (also known as leg of the autotransformer).

A monitoring and control unit is configured to receive the valuesmeasured by the sensors, then to compare these values with each otherand relative to a predetermined threshold during a predetermined period.

The disadvantage of this method for detecting opening of a phase is thatthe predetermined period must be sufficiently long and the predeterminedthreshold must be sufficiently high to allow good reliability to beachieved in terms of the detected defect.

The invention aims to provide a protection method capable of detectingopening of a phase of an autotransformer for an aircraft rapidly,precisely and with very great reliability, in simple, convenient andeconomical manner.

According to a first aspect, the object of the invention is therefore amethod for protecting a multi-phase autotransformer for an aircraft,characterized in that it includes the following steps:

-   -   receiving at least one value of current output from a first        phase of the autotransformer, at least one value of current        input into a second phase of the autotransformer and at least        one value of current output from this second phase;    -   determining, at least as a function of the values of currents        input into and output from the second phase, at least one value        of current representative of the operation of the second phase;    -   determining, at least as a function of the value of current        representative of the operation of the second phase and of the        value of current output from the first phase, at least one value        representative of the homopolar current flowing in the said        first phase;    -   comparing the value representative of the homopolar current        flowing in the said first phase with a first predetermined        threshold value at least during a first predetermined period;    -   controlling the values of current input into and/or output from        the phases of the autotransformer as a function of the said        first comparison.

To detect the opening of a phase (first phase) upstream from theautotransformer, the method according to the invention is based on ameasurement of current on this phase downstream from the autotransformerand on a measurement of current on one other phase (second phase) of theautotransformer, upstream and downstream.

By taking the upstream current and the downstream current on the otherphase into account, it is possible to create a reference byhypothesizing that this other phase is not affected by a defectcomprising opening of a phase upstream. This therefore makes it possibleto determine a value of current representative of the operation (said tobe normal) of this other phase. It is to be noted that the upstream anddownstream currents of the other phase actually represent themagnetizing current and the homopolar current flowing in this phase (orleg) of the autotransformer. This homopolar current is negligible innominal operation (in other words without a defect comprising opening ofa phase upstream).

As regards the phase for which an attempt is being made to detect if itis open upstream from the autotransformer, if only the current outputfrom this phase is taken into account it illustrates the fact, incontrast to the other phase, that the magnetizing current flowing inthis phase is negligible compared with the homopolar current flowingtherein due to the fact that this phase is (potentially) open upstreamfrom the autotransformer.

In fact, these homopolar currents flow in the turns of theautotransformer when a disequilibrium associated with the opening of aphase upstream from the autotransformer occurs. These homopolar currentsthen have an amplitude much larger than the amplitude of the magnetizingcurrent, so that the latter becomes negligible.

By virtue of the invention, the detection of false defects is avoided,and it is possible to have a predetermined threshold value much lowerthan that of the prior art as well as a period also much shorter thanthat of the prior art.

In this way the method according to the invention offers the advantageof performing measurements on the phases of the autotransformer that areprecise and rapid, thus making it possible to detect defects associatedwith opening of phases of autotransformers rapidly and with greatreliability.

The method according to the invention is therefore particularlyeffective while being simple, convenient and economical.

According to preferred simple, convenient and economical characteristicsof the method according to the invention, it additionally includes thefollowing steps:

-   -   comparing the value of current output from the first phase and        the value of current output from the second phase with a second        predetermined threshold value;    -   controlling the values of currents input into and/or output from        the phases of the autotransformer as a function of the said        second comparison.

In this way the method according to the invention offers the advantageof detecting a false defect comprising opening of a phase and, as thecase may be, of inhibiting the protection of the autotransformer.

According to preferred simple, convenient and economical characteristicsof the method according to the invention:

-   -   the step of determining at least one value of current        representative of the operation of the second phase is achieved        by a difference function of the values of currents input into        and output from the second phase, thus making it possible to        achieve even greater precision in measurement;    -   the value of current representative of the operation of the        second phase is determined by the difference between n times the        value of current input into the second phase and the value of        current output from the second phase;    -   n is equal to the transformation ratio of the autotransformer;        for example, n is equal to 2;    -   the step of determining a value representative of the homopolar        current flowing in the said first phase is achieved by a        function of summing the value of current output from the first        phase and the value of current representative of the operation        of the second phase, thus making it possible to measure the        magnetizing current, the homopolar current and the short-circuit        current even more precisely;    -   the step of controlling as a function of the said first        comparison includes zeroing the values of current input into        and/or output from each phase when the value representative of        the homopolar current flowing in the said first phase is lower        than the first predetermined threshold value at least during the        first predetermined period, thus making it possible to protect        the autotransformer;    -   the step of controlling as a function of the said second        comparison includes preventing zeroing of the values of currents        input into and/or output from each phase when the values of        currents output from the first phase and from the second phase        are each lower than the second predetermined threshold value,        thus making it possible to inhibit any zeroing of these        currents; and/or    -   the step of controlling by preventing zeroing of the values of        input and/or output currents is inhibited when the values of        currents output from the first phase and from the second phase        are each no longer lower than the second predetermined threshold        value at least during a predetermined second period, thus making        it possible once again to detect the true internal defects as        the case may be.

According to a second aspect, another object of the invention is adevice for protecting a multi-phase autotransformer for an aircraft,provided with a monitoring and control unit configured to:

-   -   receive at least one value of current output from a first phase        of the autotransformer, at least one value of current input into        a second phase of the autotransformer and at least one value of        current output from this second phase;    -   determine, at least as a function of the values of currents        input into and output from the second phase, at least one value        of current representative of the operation of the second phase;    -   determine, at least as a function of the value of current        representative of the operation of the second phase and of the        value of current output from the first phase, at least one value        representative of the homopolar current flowing in the said        first phase;    -   compare the value representative of the homopolar current        flowing in the said first phase with a first predetermined        threshold value at least during a first predetermined period;    -   control the values of currents input into and/or output from the        phases of the autotransformer as a function of the said first        comparison.

To detect the opening of a phase (first phase) upstream from theautotransformer, the device according to the invention is based on ameasurement of current on this phase downstream from the autotransformerand on a measurement of current on another phase (second phase) of theautotransformer upstream and downstream.

By taking the upstream current and the downstream current on the otherphase (in other words at the input of the turns of the primary and atthe output of the turns of the secondary) into account, it is possibleto create a reference by hypothesizing that this other phase is notaffected by a defect comprising opening of a phase upstream. Thistherefore makes it possible to determine a value of currentrepresentative of the operation (said to be normal) of this other phase.It is to be noted that the upstream and downstream currents of the otherphase actually represent the magnetizing current and the homopolarcurrent flowing in this phase (or leg) of the autotransformer. Thishomopolar current is negligible in nominal operation (in other wordswithout defect comprising opening of a phase upstream).

As regards the phase for which an attempt is being made to detect if itis open upstream from the autotransformer, if only the current outputfrom this phase is taken into account it illustrates the fact, incontrast to the other phase, that the magnetizing current flowing inthis phase is negligible compared with the homopolar current flowingtherein due to the fact that this phase is (potentially) open upstreamfrom the autotransformer.

In fact, these homopolar currents flow in the turns of theautotransformer when a disequilibrium associated with opening of a phaseupstream from the autotransformer occurs. These homopolar currents thenhave an amplitude much higher than the amplitude of the magnetizingcurrent, so that the latter becomes negligible.

By virtue of the invention, the detection of false defects is avoided,and it is possible to have a predetermined threshold value much lowerthan that of the prior art as well as a period also much shorter thanthat of the prior art.

In this way the device according to the invention offers the advantageof performing measurements on the phases of the autotransformer that areprecise and rapid, thus making it possible to detect defects related toopening of phases of autotransformers rapidly and with greatreliability.

The device according to the invention is therefore particularlyeffective while being simple, convenient and economical.

According to preferred simple, convenient and economical characteristicsof the device according to the invention:

-   -   the device is provided with measuring instruments configured to        perform measurements of values of current input into and output        from each phase of the autotransformer;    -   the said instruments are current transformers, which makes it        possible to perform very precise measurements;    -   the device is provided with at least one main contactor disposed        upstream and/or downstream from the autotransformer and        configured to act on the opening/closing of the phases upstream        and/or downstream from the autotransformer thus making it easily        possible to control the value of current input into the        autotransformer; and/or    -   the said at least one main contactor is provided with a phase        contactor on each phase of the autotransformer, thus making it        possible to trigger this control very rapidly.

According to a third aspect, another object of the invention is anaircraft equipped with at least one multi-phase autotransformer and atleast one protective device such as described above.

According to preferred simple, convenient and economicalcharacteristics, the aircraft is provided with a three-phase electricalsupply system having an rms voltage of approximately 230 V, which isconnected upstream to the autotransformer, an electrical consumingsystem having an rms voltage of approximately 115 V, which is connecteddownstream to the autotransformer, and the said autotransformer isthree-phase and configured to transform the rms voltage of approximately230 V to an rms voltage of approximately 115 V.

The explanation of the invention will now be continued by thedescription of an exemplary embodiment, provided below for illustrativebut not limitative purposes, with reference to the attached drawings,wherein:

FIG. 1 schematically represents a perspective view of an aircraftprovided in particular with an autotransformer and a protective devicein conformity with one embodiment of the invention;

FIG. 2 schematically and partially represents the electrical circuit ofthe aircraft; and

FIG. 3 is a block diagram illustrating different steps of operation of aprotective method employed by the protective device.

FIG. 1 illustrates an aircraft 1 provided with a fuselage 6, which has afront part 2 and a rear part 3, wings 4, each joined to fuselage 6 at acentral part thereof, and two engines 5, wherein each of these engines 5is fixed to a lower wall of a respective wing 4 and extends fromrespective wing 4, parallel to fuselage 6, toward front part 2 ofaircraft 1.

This aircraft 1 is additionally equipped with an electrical supplysystem 7 provided with three-phase electrical supply sources 7A, 7B, 7C(FIG. 2), traditionally formed by rotating electrical machines.

Aircraft 1 is additionally equipped with an electrical consuming system8 provided with three-phase electrical consuming sources 8 a, 8 b, 8 cand single-phase electrical consuming source 11 c, also referred to asthree-phase loads and single-phase load respectively.

Aircraft 1 is additionally equipped with at least one multi-phaseautotransformer 9 interposed between electrical supply system 7 andelectrical consuming system 8.

In the present case, the autotransformer is three-phase and isconfigured to connect supply system 7 electrically to consuming system8.

Aircraft 1 therefore has a multi-phase electrical system that in thepresent case is provided with three phases leading from electricalsupply system 7 to electrical consuming system 8 by passing throughautotransformer 9.

These three phases, denoted phase a, phase b and phase c, therefore eachhas an upstream portion extending from electrical supply system 7 to theinput of autotransformer 9, a downstream portion extending from theoutput of autotransformer 9 to electrical consuming system 8 and aninternal portion inside autotransformer 9.

Aircraft 1 is additionally equipped with a protective device, in thepresent case formed in particular by a monitoring and control unit 10connected electrically at least to each of the phases upstream fromautotransformer 9 and downstream from autotransformer 9.

This monitoring and control unit 10 is configured to receiverepresentative information such as the values of current obtained bymeasurements performed on each of these phases.

This monitoring and control unit 10 is additionally configured toprocess the information that it receives, to run data comparisons and tocontrol, as a function of these comparisons, input currents IA, IB, ICand output currents Ia, Ib, Ic of autotransformer 9.

FIG. 2 illustrates the electrical circuit connecting the electricalsupply system to the electrical consuming system by way ofautotransformer 9.

Three-phase electrical supply sources 7A, 7B and 7C are interconnectedat a first neutral point 22, which is also referred to as the neutralpoint of the upstream system and is connected to a neutral linerepresented by the symbol N.

These electrical supply sources 7A, 7B and 7C deliver electrical energythat in the present case has a three-phase a.c. rms voltage of 230 V.

The three phases a, b, c draw their source from upstream supply system 7and are directed toward autotransformer 9.

Autotransformer 9 has a magnetic circuit 12, which is closed and in thepresent case is provided with three legs, respectively a first leg 13, asecond leg 14 and a third leg 15. These legs 13, 14 and 15 respectivelyform the core of a respective phase.

First leg 13 forms the core of phase a, second leg 14 forms the core ofphase b and third leg 15 forms the core of phase c.

Autotransformer 9 additionally has primary coils and common coilsforming in particular the secondary coils.

In fact, in an autotransformer, the secondary is formed by a part of theprimary winding so that the supply current (primary) passes though thecorresponding primary coil and the corresponding common coil in itsentirety, whereas the output current (secondary) passes through only thecorresponding common coil, by virtue of a tap at a given point thereofthat determines the output of the secondary. In this way only theprimary currents pass through the primary coils whereas the algebraicsum of the primary and secondary currents passes through the commoncoils.

In the present case, autotransformer 9 has, on phase a (also known asfirst phase), a first primary coil 16, also known as primary coil ofphase a, a first common coil 19, also known as common coil of phase a,and a first tap point 25 disposed between first primary coil 16 andfirst common coil 19.

It is understood that these first primary and common coils 16 and 19respectively are formed by turns wound around core 13 of phase a.

The input current on the portion of phase a internal to autotransformer9 is denoted IA and the output current is denoted la.

Input current IA is obtained from supply source 7A, and current Ia isdirected toward load 8 a.

In the present case, autotransformer 9 has, on phase b (also known assecond phase), a second primary coil 17, also known as primary coil ofphase b, a second common coil 20, also known as common coil of phase b,and a second tap point 26 disposed between second primary coil 17 andsecond common coil 20.

It is understood that these second primary and common coils 17 and 20respectively are formed by turns wound around core 14 of phase b.

The input current on the portion of phase b internal to autotransformer9 is denoted IB and the output current is denoted Ib.

Input current IB is obtained from supply source 7B, and current Ib isdirected toward load 8 b.

In the present case, autotransformer 9 has, on phase c (also known asthird phase), a third primary coil 18, also known as primary coil ofphase c, a third common coil 21, also known as common coil of phase c,and a third tap point 27 disposed between third primary coil 18 andthird common coil 21.

It is understood that these third primary and common coils 18 and 21respectively are formed by turns wound around core 15 of phase c.

The input current on the portion of phase c internal to autotransformer9 is denoted IC and the output current is denoted Ic.

Input current IC is obtained from supply source 7C, and current Ic isdirected toward load 8 c.

The three common coils 19, 20 and 21 are interconnected at a secondneutral point 23, referred to as neutral point of autotransformer 9.

In the present case, autotransformer 9 therefore has coupling of thestar-star type, meaning that it is connected to the neutral upstream anddownstream.

A three-phase system is formed by three alternating variables of thesame nature and same frequency. Three sinusoidal variables form anequilibrated system if they have the same rms values and if they have aregular phase shift between them. However, if the system isdisequilibrated, for example because of a defect upstream and/ordownstream from autotransformer 9, the operation of the circuit cannotbe analyzed directly because the value to be allocated to the impedanceof the various elements is not known. It is then considered that thedisequilibrated system is formed by three systems, the direct system,the inverse system and the homopolar system.

It is considered that all the systems represented by vectors grouped instar configuration have the same direct and inverse components.Consequently, it is considered that only the homopolar component, inother words the homopolar current, is capable of flowing in the phasesof autotransformer 9 while having a non-negligible amplitude when adefect occurs. The three currents IAaN, IBbN and ICcN coming from theneutral and flowing respectively in the phases are represented in FIG. 2as extending from neutral line N at the second neutral point 23.

Autotransformer 9 has a transformation ratio between the input voltageand the output voltage that permits it to deliver, at the output, athree-phase a.c. voltage having an rms value of approximately 115 V.

This transformation ratio is equal to the ratio of the number of turnsin which the secondary current flows to the number of turns in which theprimary current flows, or to the inverse, since the autotransformer isreversible and so also is the detection method.

The electrical consuming system is formed in FIG. 2 from threethree-phase electrical loads 8 a, 8 b and 8 c, all three of which areinterconnected at one end and which are additionally connected atanother end to the respective phases.

These loads 8 a, 8 b and 8 c are therefore electrically supplied bycurrents Ia, Ib and Ic respectively.

In the present case, the electrical consuming system is additionallyformed by a single-phase load 11 c connected on the one hand to neutralline N and on the other hand to phase c at a fourth tap point 24, towhich three-phase load 8 c is also connected.

This fourth tap point 24 is also referred to as tap point for thethree-phase and single-phase loads on phase c.

Monitoring and control unit 10 is provided with a microprocessor (notrepresented) equipped with a memory (not represented), in particularnonvolatile, permitting it to load and store information, and with asoftware routine which, when executed in the microprocessor, permits theemployment of a detection method according to the invention.

This nonvolatile memory is, for example, of ROM type (“Read OnlyMemory”).

Monitoring and control unit 10 is additionally provided with a memory(not represented), in particular volatile, making it possible to storedata in memory during execution of the software and employment of themethod.

This volatile memory is, for example, of RAM or EEPROM type(respectively “Random Access Memory” and “Electrically ErasableProgrammable Read Only Memory”).

This monitoring and control unit 10 may be provided, for example, with amicrocontroller or an ASIC (“Application-Specific Integrated Circuit”).

This monitoring and control unit 10 is configured such that it candynamically receive information representative, directly in the presentcase, of measured values of input currents IA, IB and IC and of outputcurrents Ia, Ib and Ic.

The protective device is additionally provided with current transformers28, 29, 30, 31, 32 and 33, which are separated from autotransformer 9 inorder to be in a less constraining thermal environment.

In the present case these current transformers 28, 29, 30, 31, 32 and 33are of the nanocrystalline core type and they measure the values ofcurrents IA, IB, IC, Ia, Ib and Ic.

In the present case, current transformers 28 to 33 (which are actuallyspecific measuring transducers) are directly integrated into monitoringand control unit 10 and are each connected respectively to thecorresponding phase upstream and/or downstream from autotransformer 9.

In the present case, current transformers 28, 29, 30, 31, 32 and 33 areconnected respectively to the upstream portions of the phases and to thedownstream portions of phases a, b and c.

This monitoring and control unit 10 therefore directly receives valuesof current input into each phase of autotransformer 9 and output fromeach phase of this autotransformer 9.

The protective device is also provided with main contactors 34 and 35,with which monitoring and control unit 10 is configured to communicate,and in particular to send orders.

Contactor 34 is referred to as upstream main contactor, because it isprovided with three phase contactors, respectively a phase a contactor36 disposed on the upstream portion of phase a, a phase b contactor 37disposed on the upstream portion of phase b, and a phase c contactor 38disposed on the upstream portion of phase c.

These upstream phase contactors 36 to 38 permit the opening and/orclosing of the respective phase on which each contactor is mounted.

These upstream phase contactors 36 to 38 are controlled by upstream maincontactor 34.

Contactor 35 is referred to as downstream main contactor, because it isprovided with three phase contactors, respectively a phase a contactor39 disposed on the downstream portion of phase a, a phase b contactor 40disposed on the downstream portion of phase b, and a phase c contactor41 disposed on the downstream portion of phase c.

These downstream phase contactors 39 to 41 permit the opening and/orclosing of the respective phase on which each contactor is mounted.

These downstream phase contactors 39 to 41 are controlled by upstreammain contactor 35.

Autotransformer 9 makes it possible to reduce the weight in aircraft 1and it ensures compatibility with the stock of electrical machines onthe ground that are configured to deliver an rms voltage ofapproximately 230 V.

Since autotransformer 9 is connected to neutral line N, it is able tocontinue supplying three-phase loads 8 a, 8 b and 8 c even when one ofthese phases is open on its upstream portion, in other words on the 230V side.

This abnormal operation may cause overheating of autotransformer 9,especially when three-phase electrical loads 8 a, 8 b and 8 c aredemanding maximum power.

It is necessary to have a protection method capable of detecting openingof a phase upstream from autotransformer 9 rapidly and particularlyreliably, with the goal of avoiding detection of false defects.

FIG. 3 is a block diagram of steps permitting protection ofautotransformer 9 by detection of opening of any one of the phases; andby inhibiting this detection when autotransformer 9 is not supplying anyelectrical load, on the 115 V side.

For this purpose, current transformers 28 to 33 respectively measure thevalues of input current IA, IB, IC and of output current Ia, Ib, Ic, andmonitoring unit of two controls 10 receives these values of current instep 100.

From these values of current, monitoring and control unit 10 determines,in step 101, an instantaneous value of current representative of theoperation of corresponding phase IFa, IFb and IFc by calculating, foreach of these values, the value of the result of subtraction between twotimes the instantaneous current input into the corresponding phase andthe instantaneous current output from this same phase.

The calculations performed in step 101 by monitoring and control unit 10are formulated in the following manner:

$\left\{ \begin{matrix}{I_{Fa} = {{{2 \times I_{A}} - I_{a}}}} \\{I_{Fb} = {{{2 \times I_{B}} - I_{b}}}} \\{I_{Fc} = {{{2 \times I_{C}} - I_{c}}}}\end{matrix}\quad \right.$

These values of current representative of the operation of each phaseactually measure, in each leg 13 to 15 of autotransformer 9, themagnetizing current in this phase as well as the homopolar current,which is negligible during nominal operation but non-negligible in caseof opening of a phase upstream from autotransformer 9.

Monitoring and control unit 10 then determines, in step 102, valuesrepresentative of the homopolar current Idet-a, Idet-b and Idet-cflowing in each of the phases for the corresponding phases bycalculating, for each of these values, the absolute value of the sumamong the output current on the corresponding phase and the value ofcurrent representative of the operation of this phase determinedpreviously in step 101.

The calculations performed in step 102 by monitoring and control unit 10are formulated in the following manner:

$\left\{ \begin{matrix}{I_{\det - a} = {{I_{a} - I_{Fb}}}_{rms}} \\{I_{\det - b} = {{I_{b} - I_{Fc}}}_{rms}} \\{I_{\det - c} = {{I_{c} - I_{Fa}}}_{rms}}\end{matrix}\quad \right.$

In this calculation step, if only the current output from thecorresponding phase for which an attempt is being made to detect if itis open upstream from autotransformer 9 is taken into account, itillustrates the fact that the magnetizing current flowing in this phaseis negligible compared with the homopolar current flowing therein due tothe fact that this corresponding phase is (potentially) open upstreamfrom autotransformer 9. In addition, the comparison is carried out withthe value of current representative of the operation of another phase,which itself is considered, by hypothesis, as operating normally, inother words without any defect comprising opening of a phase upstream.

Monitoring and control unit 10 then compares, in step 104, each of thevalues representative of homopolar current Idet-a, Idet-b and Idet-cflowing in the corresponding phase with a first predetermined thresholdvalue S1 previously received by monitoring and control unit 10 in a step103.

To detect if one of the phases is open, monitoring and control unit 10verifies, in step 104, if the value representative of the homopolarcurrent flowing in the corresponding phase is lower than this firstthreshold value S1, which in the present case is equal to approximately10 A (in rms value).

The calculations performed by monitoring and control unit 10 in step 104are formulated in the following manner:

$\left\{ \begin{matrix}{I_{\det - a} < {S\;{1?}}} \\{I_{\det - b} < {S\;{1?}}} \\{I_{\det - c} < {S\;{1?}}}\end{matrix}\quad \right.$

When the value Idet-a representative of the homopolar current flowing inphase a is lower than S1, then monitoring and control unit 10 verifies,in step 105, that this result is confirmed after a first predeterminedperiod D1, which in the present case is equal to approximately 40 ms.

If this is the case, in other words if this result is confirmed afterthe period D1, then monitoring and control unit 10 makes the decision toprotect autotransformer 9 in step 106, because phase a is then open. Forthis purpose, monitoring and control unit 10 sends an order to open theupstream portion of phase a at upstream main contactor 34, whichcomplies with this order by controlling the opening of phase a contactor36 but also the opening of phase b and c contactors 37 and 38.

In this way autotransformer 9 is protected from a short circuit on itsportions of phases a, b and c.

When the value Idet-b representative of the homopolar current flowing inphase b is lower than S1, then monitoring and control unit 10 verifies,in step 107, that this result is confirmed after this firstpredetermined period D1, which in the present case is equal toapproximately 40 ms.

If this is the case, in other words if this result is confirmed afterthe period D1, then monitoring and control unit 10 makes the decision toprotect autotransformer 9 in step 108, because phase b is then open. Forthis purpose, monitoring and control unit 10 sends an order to open theupstream portion of phase b at upstream main contactor 34, whichcomplies with this order by controlling the opening of phase b contactor37 but also the opening of phase a and c contactors 36 and 38.

In this way autotransformer 9 is protected from a short circuit on itsportions of phases a, b and c.

When the value Idet-c representative of the homopolar current flowing inphase c is lower than S1, then monitoring and control unit 10 verifies,in step 109, that this result is confirmed after this firstpredetermined period D1, which in the present case is equal toapproximately 40 ms.

If this is the case, in other words if this result is confirmed afterthe period D1, then monitoring and control unit 10 makes the decision toprotect autotransformer 9 in step 110, because phase c is then open. Forthis purpose, monitoring and control unit 10 sends an order to open theupstream portion of phase c at upstream main contactor 34, whichcomplies with this order by controlling the opening of phase c contactor38 but also the opening of phase a and b contactors 36 and 37.

In this way autotransformer 9 is protected from a short circuit on itsportions of phases a, b and c.

In this way autotransformer 9 is protected from an internal shortcircuit on all these phases.

In parallel with steps 101 to 110, monitoring and control unit 10 isconfigured to compare, in step 112, the value of current output fromeach phase 1 a, 1 b and 1 c in absolute value with a secondpredetermined threshold value S2 previously received by monitoring andcontrol unit 10 in a step 111.

In the present case, second predetermined threshold value S2 is equal to20 A in rms value.

The comparison is carried out by monitoring and control unit 10 bycalculating if the absolute value of the current output from each phaseis lower than S2.

Expressed otherwise, the calculations performed in step 112 may bewritten in the following manner:

$\left\{ \begin{matrix}{{I_{a}}_{rms} < {S\; 2}} \\{and} \\{{I_{b}}_{rms} < {S\; 2}} \\{and} \\{{I_{c}}_{rms} < {S\;{2?}}}\end{matrix}\quad \right.$

If such is the case, in other words if the absolute values of currentsIa, Ib and Ic are each lower than S2, then monitoring and control unit10 switches to step 113, which is a step of inhibition of steps 106, 108and 110 corresponding to protection of autotransformer 9.

This inhibition corresponds to monitoring of the values of currentsinput into autotransformer 9, such that zeroing thereof is prevented.

The comparison performed in step 112 for each of the currents outputfrom autotransformer 9 makes it possible to detect when thisautotransformer 9 is not supplying any electrical load on the 115 Vside. If this comparison is positive, it means that autotransformer 9 isnot supplying any load on the consuming system.

As the case may be, steps 101 to 105 employed by monitoring and controlunit 10 would be able to detect the presence of a false defect, and itis necessary to inhibit steps 106, 108 and 110 so as not to protectautotransformer 9 because of such a false defect.

Monitoring and control unit 10 then performs, in step 114, the samecomparison as in step 112, and it reiterates this comparison in the casethat the latter is positive, while maintaining inhibition of steps 106,108 and 110.

In the case that this comparison is negative, monitoring and controlunit 10 verifies that this negative result of comparison is confirmedafter a second predetermined period D2, in step 115.

In the present case, this second predetermined period D2 isapproximately equal to 20 ms.

When the negative result of the comparison performed in step 114 isconfirmed in step 115 by monitoring and control unit 10, the latterdisinhibits steps 106, 108 and 110 in step 116, since in this caseautotransformer 9 is supplying electrical loads on the electricalconsuming system.

After these steps 106, 108 and 110 for protection of autotransformer 9have been disinhibited, steps 104 to 110 may be employed, as may steps102 and 112 to 116 on the basis of dynamic measurements performed instep 100.

The values of S1, D1, S2 and D2 are typical of a three-phaseautotransformer having a nominal active power of approximately 60 kVA.

Of course, these values of S1, D1, S2 and D2 may change as a function ofthe circumstances, in particular as a function of the autotransformerbeing used.

In variants that are not illustrated:

-   -   the current transformers are not installed in the monitoring and        control unit but instead are installed directly on the phases or        elsewhere in the device;    -   the monitoring and control unit does not send open/close orders        upstream from the autotransformer on the phases, but instead        sends them downstream from the autotransformer on the phases;    -   the upstream and downstream main contactors are not separate        from the monitoring and control unit, but instead are directly        integrated into it;    -   more generally, the value of the magnetizing current is        determined on the basis in particular of n times the current        input to a phase, where n corresponds to the transformation        ratio of the autotransformer; and/or    -   the autotransformer has a transformation ratio different from        that (equal to 2) which makes it possible to transform a        three-phase a.c. voltage of 230 V into a three-phase a.c.        voltage of 115 V, and these voltage values may well be        different;    -   the transformation ratio of the autotransformer is not equal to        2 but instead is equal to 0.5 in the case in which the        autotransformer is used reversibly, for example with the stock        of electrical machines on the ground.

It is recalled more generally that the invention is not limited to theexamples described and represented.

The invention claimed is:
 1. A method for protecting a multi-phaseautotransformer for an aircraft, the method comprising: receiving atleast one value of current output from (I_(a), I_(b), I_(c)) a firstphase of the autotransformer, at least one value of current input into(I_(A), I_(B), I_(C)) a second phase of the autotransformer and at leastone value of current output from (I_(a), I_(b), I_(c)) the second phase;determining, at least as a function of the values of currents input into(I_(A), I_(B), I_(C)) and output from (I_(a), I_(b), I_(c)) the secondphase, at least one value of current representative of the operation(I_(Fa), I_(Fb), I_(Fc)) of the second phase; determining, at least as afunction of the value of current representative of the operation(I_(Fa), I_(Fb), I_(Fc)) of the second phase and of the value of currentoutput from (I_(a), I_(b), I_(c)) the first phase, at least one valuerepresentative of the homopolar current (I_(det-a),I_(det-b),I_(det-c))flowing in the first phase; comparing the value representativeof the homopolar current (I_(det-a), I_(det-b),I_(det-c)) flowing in thefirst phase with a first predetermined threshold value (S1) at leastduring a first predetermined period (D1); and controlling the values ofcurrents input into (I_(A), I_(B), I_(C)) or output from (I_(a), I_(b),I_(c)) the phases of the autotransformer as a function of the comparing.2. The method according to claim 1, further comprising: second comparingthe value of current output from (I_(a), I_(b), I_(c)) the first phaseand the value of current output from (I_(a), I_(b), I_(c)) the secondphase with a second predetermined threshold value (S2); and controllingthe values of currents input into (I_(A), I_(B), I_(c))or output from(I_(a), I_(b), I_(c)) the phases of the autotransformer as a function ofthe second comparing.
 3. The method according to claim 1, wherein thestep of determining at least one value of current representative of theoperation (I_(Fa),I_(Fb), I_(Fc),) of the second phase is achieved by adifference function of the values of current input into (I_(A), I_(B),I_(c)) and output from (I_(a), I_(b), I_(c)) the second phase.
 4. Themethod according to claim 3, wherein the value of current representativeof the operation (I_(Fa), I_(Fb), I_(Fc)) of the second phase isdetermined by the difference between n times the value of current inputinto (I_(A), I_(B), I_(c)) the second phase and the value of currentoutput from (I_(a), I_(b), I_(c)) the second phase.
 5. The methodaccording to claim 4, wherein n is equal to transformation ratio of theautotransformer, n being equal to
 2. 6. The method according to claim 1,wherein the step of determining a value representative of the homopolarcurrent (I_(det-a), I_(det-b), I_(det-c)) flowing in the first phase isachieved by a function of summing the value of current output from(I_(a),I_(b), I_(c)) the first phase and the value of currentrepresentative of the operation (I_(Fa), I_(Fb), I_(Fc)) of the secondphase.
 7. The method according to claim 2, wherein the step ofcontrolling as a function of the comparing includes zeroing the valuesof current input into (I_(A), I_(B), I_(c)) or output from (I_(a),I_(b), I_(c)) each phase when the value representative of the homopolarcurrent (I_(det-a), I_(det-b), I_(det-c)) flowing in the first phase islower than the first predetermined threshold value (S1) at least duringthe first predetermined period (D1).
 8. The method according to claim 7wherein the step of controlling as a function of the second comparingincludes preventing zeroing of the values of currents input into (I_(A),I_(B), I_(c)) or output from (I_(a), I_(b), I_(c)) each phase when thevalues of currents output from (I_(a), I_(b), I_(c)) the first phase andthe second phase are lower than the second predetermined threshold value(S2).
 9. The method according to claim 8, wherein the step ofcontrolling by preventing zeroing of the values of input (I_(A), I_(B),I_(c)) or output (I_(a), I_(b), I_(c)) currents is inhibited when thevalues of currents output from (I_(a), I_(b), I_(c)) the first phase andthe second phase each are no longer lower than the second predeterminedthreshold value (S2) at least during a predetermined second period (D2).10. A device for protecting a multi-phase autotransformer for anaircraft, comprising: a monitoring and control unit configured toreceive at least one value of current output from (I_(a), I_(b), I_(c),)a first phase of the autotransformer, at least one value of currentinput into (I_(A), I_(B), I_(c)) a second phase of the autotransformerand at least one value of current output from (I_(a), I_(b), I_(c)) thesecond phase; determine, at least as a function of the values ofcurrents input into (I_(A), I_(B), I_(C)) and output from (I_(a), I_(b),I_(c)) the second phase, at least one value of current representative ofthe operation (I_(Fa), I_(Fb), I_(Fc)) of the second phase; determine,at least as a function of the value of current representative of theoperation (I_(Fa), I_(Fb), I_(Fc)) of the second phase and of the valueof current output from (I_(a), I_(b), I_(c)) the first phase, at leastone value representative of the homopolar current (I_(det-a), I_(det-b),I_(det-c)) flowing in the first phase; compare the value representativeof the homopolar current (I_(det-a), I_(det-b), I_(det-c)) flowing inthe first phase with a first predetermined threshold value (S1) at leastduring a first predetermined period (D1); and control the values ofcurrents input into (I_(A), I_(B), I_(C)) or output from (I_(a), I_(b),I_(c)) the phases of the autotransformer as a function of comparison ofthe value representative of the homopolar current (I_(det-a), I_(det-b),I_(det-c)) flowing in the first phase with the first predeterminedthreshold value (S1).
 11. The device according to claim 10, furthercomprising measuring instruments configured to perform measurements ofvalues of currents input into (I_(A), I_(B), I_(C)) and output from(I_(a), I_(b), I_(c)) each phase of the autotransformer.
 12. The deviceaccording to claim 10, further comprising at least one main contactordisposed upstream or downstream from the autotransformer and configuredto act on the opening or closing of the phases upstream or downstreamfrom the autotransformer.
 13. The device according to claim 12, whereinthe at least one main contactor is provided with a phase contactor oneach phase of the autotransformer.
 14. An aircraft comprising: at leastone multi-phase autotransformer; and at least one protective deviceconfigured to receive at least one value of current output from (I_(a),I_(b), I_(c)) a first phase of the autotransformer, at least one valueof current input into (I_(A), I_(B), I_(C)) a second phase of theautotransformer and at least one value of current output from (I_(a),I_(b), I_(c)) the second phase, determine, at least as a function of thevalues of currents input into (I_(A), I_(B), I_(C)) and output from(I_(a), I_(b), I_(c)) the second phase, at least one value of currentrepresentative of the operation (I_(Fa), I_(Fb), I_(Fc)) of the secondphase, determine, at least as a function of the value of currentrepresentative of the operation (I_(Fa), I_(Fb), I_(Fc)) of the secondphase and of the value of current output from (I_(a), I_(b), I_(c)) thefirst phase, at least one value representative of the homopolar current(I_(det-a), I_(det-b), I_(det-c)) flowing in the first phase, comparethe value representative of the homopolar current (I_(det-a), I_(det-b),I_(det-c)) flowing in the first phase with a first predeterminedthreshold value (S1) at least during a first predetermined period (D1),and control the values of currents input into (I_(A), I_(B), I_(C)) oroutput from (I_(a), I_(b), I_(c)) the phases of the autotransformer as afunction of comparison of the value representative of the homopolarcurrent (I_(det-a), I_(det-b), I_(det-c)) flowing in the first phasewith the first predetermined threshold value (S1).
 15. The aircraftaccording to claim 14, further comprising a three-phase electricalsupply system having an rms voltage of approximately 230 V, which isconnected upstream to the autotransformer, an electrical consumingsystem having an rms voltage of approximately 115 V, which is connecteddownstream to the autotransformer, wherein the autotransformer isthree-phase and configured to transform the rms voltage of approximately230 V to an rms voltage of approximately 115 V.